Abstract: Real-time simulation of ex-core nuclear instrumentation system is an indispensable part of nuclear power plant（NPP） full-scope training simulator.The simulation method,which is based upon the theory of measurement,is introduced in the paper.The fitting formula between the measured data and the three-dimensional neutron flux distribution in the core is established.The fitting parameter is adjusted according to the reactor physical calculation or the experiment of power calibration.The simulation result shows that the method can simulate the ex-core neutron instrumentation system accurately in real-time and meets the needs of NPP full-scope training simulator.